Field of the Invention
The present invention relates to a supervisory control system for continuous drying of moist solid products to reduce the moisture content thereof, and more particularly to the use of distributed process controls utilizing simple function blocks for tight control of the temperature and in turn of the residual level of moisture in the dried end product.
The drying process accounts for up to about 10% of all industrial energy usage. Control of industrial drying process operations has been less improved than is economically desirable or feasible, yet advanced control methods using distributed control systems might well be implemented therefore with a concomitant attractive return on investment.
Dryers are widely used in process industries such as pulp and paper, food, chemicals, building materials, metals, textiles, pharmaceuticals, ceramics and agriculture. The conventional types of dryers most commonly used are fluidized bed, kiln, rotary, conveyor, solar, batch, pan, spray, etc. dryers.
As in any processing operation, the goal of pertinent control strategies and methods of operating a continuous dryer is high profitability. This profitability can be improved potentially in terms of reduced energy costs, increased productivity and improved product quality.
Traditionally, the outlet dry bulb temperature T.sub.0 of the drying agent (which is normally air) leaving the dryer is controlled, i.e. the process is monitored in terms of the measurement of the exhaust air temperature. Load variations are handled by modifying the inlet dry bulb temperature T.sub.i of the hot drying medium (air) entering the dryer. However this approach generally causes underdrying or overdrying, due to changing product load conditions, which degrades the dryer performance even though the temperatures are adequately controlled. Indeed, humidity must be controlled accurately to cope with the normally encountered variations in mass, flow and in moisture content of the starting product entering the dryer.
The main incentives for precise control of humidity in dryers in this regard are:
1. Reduced energy usage per unit weight product throughput. PA1 2. Increased production rate for a given size dryer installation. PA1 3. Increased profit from increased moisture sold as product where appropriate. PA1 4. Reduced chance of fire. PA1 5. Reduced production of defective products. PA1 6. Reduced particle emission.
Generally, higher efficiency is obtained by observing such conditions as high temperature and low humidity which help increase the ability of the hot air to pick up moisture from the product during drying, and low exhaust volume or outlet air flow which represents a reduced energy and equipment cost. However, the necessary constraints of product quality, e.g. freedom from scorching, and excessive heat loss must be considered when the use of increased temperatures for the drying operation are proposed.
In the case of adiabatic continuous drying of wet solid products with a gaseous drying medium such as air, atmospheric pressure (14.7 psi), i.e. at generally constant pressure, in which the product moisture is evaporated from the product top surface, the product temperature remains generally constant throughout its travel e.g. on a conveyor through the dryer and is approximately the same as the wet bulb temperature T.sub.w of the drying medium . As the hot drying medium, which has a relatively low relative humidity RH and a relatively high inlet dry bulb temperature T.sub.i when it enters the dryer, takes on moisture from the wet product, the relative humidity of the medium increases and its temperature decreases. Thus, upon giving up heat to the moisture in the evaporation process, the drying medium is cooled to the relatively low outlet dry bulb temperature T.sub.0.
However, ignoring normal heat losses the heat content (enthalpy) of the gaseous drying medium, e.g. air, is considered to be the same at the inlet and outlet ends of the gas flow path of the dryer since the heat given up by the drying medium is still contained in the taken up moisture. This can be theoretically measured by a wet bulb thermometer since we have constant heat the process will have a correspondingly constant wet bulb temperature T.sub.w. On the other hand, the reduction in the dry bulb temperature of the drying medium from T.sub.i to T.sub.o is proportional to the amount of water which is evaporated from the product.
The temperature difference between the drying medium and the product at the dryer inlet increases with increasing load but such temperature difference decreases at the dryer outlet since the product temperature generally follows the constant wet bulb temperature T.sub.w whereas the drying medium decreases from the higher inlet dry bulb temperature T.sub.i to the lower outlet dry bulb temperature T.sub.0 as it takes on moisture from the product under the adiabatic conditions. Hence, with an increase in product load underdrying is prone to occur and the end product may exceed the maximum moisture limit or product reject level set for the product. This is but one of the control problems encountered in drying operations.
Such temperature difference between the drying medium and the product constitutes the driving force (T.sub.i -T.sub.w) at the inlet end and the driving force (T.sub.0 -T.sub.w) at the outlet end for driving (evaporating) moisture from the product.
Psychromatric charts are available which suitably show the drying temperature of the medium plotted against the weight of the water vapor or humidity removed in the drying process per unit weight of dry medium (air), giving related wet bulb temperature data as well, usually in terms of a given constant T.sub.w relative to the humidity increase between that at T.sub.i and that at T.sub.0 under adiabatic (constant enthalpy) conditions at constant atmospheric pressure.
The prior art contains many proposals for effecting and controlling continuous drying operations such as the continuous drying of wet solids.
Thus, Threokelv, J. L., "Thermal Environmental Engineering", Chap. 18, 1962, Prentice-Hall, describes the dynamics of continuous drying of wet solids.
Fadum, O., and Shinsky, G., "Saving Energy Through Better Control of Continuous Batch Dryers", Control Engineers, March 1980, pp. 69-72, describes a control system for saving energy in which the exit gas (air) temperature is controlled by the control set point adjustment of the hot gas entering the dryer, involving a cascade loop. Based on dryer types and inferential measurement of the wet bulb temperature of the hot gases in turn the exit gas temperature setting is modified. A positive feedback instability is avoided by a low gain and by a lag network. The psychromatric properties of the air are taken into account. Linearization is performed to approximate the thermodynamic properties of the air. Constant air flow is considered for a simplified feedback control. Scorching of the product is avoided by limiting the dryer inlet temperature and controlling the feed rate of the product for a desired product moisture.
Zagorzycki, P. E., "Automatic Humidity Control of Dryers", Chemical Engineering Progress (C.E.P.), April, 1983, pp. 66-70, discusses a control system in which the dew point temperature of the exhaust gases (air) exiting from the dryer is measured to control the air flow damper at the exit. As dew point is an indication of moisture, the exhaust flow can dictate the dew point by controlling the supply of outside air, i.e. dry air into the dryer.
Bertin, R., and Srour, Z., "Search Methods Through Simulation for Parameter Optimization of Drying Process", Drying 1980, Vol. 2, pp. 101-106, Proceedings of the 2nd Intl. Symp. on Drying, July 6-9, 1980, Montreal, Hemisphere Publ. concerns a proposal in which the dryer is modeled and the operation optimized by using an extensive amount of computations. A continuous system is transformed into a discrete system by increasing the number of variables and performing integration by a predictor corrector method. Furthermore, weighted least squares estimates are utilized for model fitting. For optimization, steepest descent and similar methods are utilized. The methods utilized high level computer languages. The goal of this work is to provide optimum steady state operation for capacity production versus tray loading for optimum drying as regards product moisture.
Moden, P. E., and Nybrant, T. "Adaptive Control of Rotary Drum Driers", Digital Computer Applications to Process Control, Proceedings of the 6th I.F.A.C./I.F.I.P. Conf., 1980, pp. 355-361, discusses a system in which an adaptive control is implemented to control the moisture of the product in a rotary drum dryer. The method utilizes extensive computation with high level computer language. The control, although advanced, is restricted to feedback control of moisture.
Waller, M., and Curtis, S., "Energy Management for Drying Systems By a Computer-Based Decision Aid", Proceedings of the 2nd Into. Symp. on Drying, July 6-9, 1980, pp. 495-499, Montreal, Hemisphere Publ., concerns a system in which optimization with respect to energy is treated. However, this method also uses high level computer languages and deals with the steady state operation to guide the operators.
U.S. Pat. No. 4,471,027, issued Oct. 2, 1984, to Kaya, A. and Moss, W. H., concerns the optimum control of cooling tower water temperature by function blocks involving wet bulb temperature estimation.
Much room for improvement in profitability results exists in drying operations in terms of reduced energy costs, increased productivity and improved product quality, as compared to the results achievable with the above described known proposals.